Modern heat engines with open working cycles, such as diesel engines, auto engines and gas turbines, are internal combustion engines. Internal combustion engines use air as the working fluid and oxidize the fuel fed to the engine as a source of heat. Gases that form as a result of fuel combustion must not contain solid combustion products because solid contaminants are detrimental to the mechanical systems used for the engine. To avoid the problems associated with solid contaminants, gaseous or liquid light hydrocarbons are employed as fuels in internal combustion engines. Attempts to use solid substances as fuel for these engines have proven unsuccessful because it is extremely difficult to remove solid particles from the combustion gases. Changing the solid fuel to a combustible gas using gasifiers results in additional complexity and loss of efficiency. Gasification also increases size, weight and cost of the equipment.
Many rural regions of the world are not served by public or quasi-public utility energy suppliers of electricity and fuel gas. In some cases these needs are presently served by portable internal combustion engine generator units fueled by highly refined liquid fuels. However, the availability and cost of these fuels may limit the use and economy of such generator units. Since internal combustion engines require a clean burning fuel source for compatability with precisely machined mechanisms, they cannot be operated directly using fuel sources such as wood, biomass, solar and geothermal energy, which are generally inexpensive and available worldwide.
External combustion closed cycle engines that use air as the working fluid have been developed in order to burn solid fuel external to the cycle. Air was selected as the working fluid because it is available and benign. Closed cycle engines using air as the working fluid have met with only limited success because air has a low heat transfer coefficient and because heat exchangers that use air are generally large in size and have significant parasitic pressure losses. Other inert working fluids have been used with limited success.
U.S. Pat. No. 5,392,606, which is incorporated by reference herein, discloses a closed circulation system for a chemically active working fluid with improved efficiency over earlier closed systems. In the thermochemical converter described in U.S. Pat. No. 5,392,606, a chemically active working fluid having a relatively large molecular weight is pressurized in a compressor such that the temperature of the working fluid and the pressure of the working fluid increases. The working fluid is further heated in a recuperator with the heat of the turbine exhaust gases, and after the recuperator, is heated in a heater with heat from an external combustion chamber to the maximum temperature of the cycle. This heating results in chemical dissociation to form a mixture of lower molecular weight components. The working fluid is introduced to a turbine where it undergoes adiabatic expansion to the lower pressure cycle and provide shaft work. The working fluid then goes to the recuperative heat exchanger where it is cooled and partially recombined, releasing heat to the high pressure flow on the other side of the recuperator. The working fluid the goes to a boiler, where cooling and recombination of the active working fluid continues with the release of sensible heat from cooling gases and chemical reaction heat from recombination of the working fluid. This heating causes evaporation of water in the boiler. After the boiler, the boiler fluid passes through a condenser and through a throttle to an evaporator, where it is evaporated with the absorption of heat (cooling effect) and enters the compressor.
The thermochemical converter of U.S. Pat. No. 5,392,606 represents an improvement over closed cycle systems that use conventional working fluids. However, there is a strong demand within the industry and among consumers for further improvements to thermochemical converters to increase efficiency for power generation of general applicability.